The lesson theme
You can teach anything you wish
You are not limited by the 'theme' topics listed on the examiners form - they are simply there to help the examiner mark your performance.
It's worth reiterating here that whatever you are teaching you will need to deal with whatever arises during the lesson.
Example 1: If you are doing a left turn lesson with a beginner and he/she drives too close to parked cars, that error cannot be ignored simply because it's not the main topic of the lesson - you will need to deal with it.
Example 2: If you are giving a motorway lesson to a full licence holder and on the way to the motorway he/she has other errors, for example mirrors, signals or following too close, you will need to sort them - you can't ignore them simply because they did not happen on the motorway.
Next: More about the lesson theme...